The present invention concerns a temperature compensated time base, and a watch incorporating the time base. More precisely the time base is of the type in which compensation for the effect of temperature on the frequency produced by the time base is effected by using two piezoelectric resonators to form said time base, usually quartz resonators.
It is well known that present-day electronic watches are very accurate. However, in such watches, the time base is formed by an oscillator including a quartz resonator. The frequency of the oscillator, which is generally of the order of 32 kHz, is temperature dependent. In consequence, the resonator has a very precise frequency for a given temperature but when the temperature changes the frequency of the resonator also changes. For example, in the case of a quartz time base in which the resonator gives the reference frequency at a temperature of 25.degree. C., over a range from 0.degree. to 50.degree. C., the operating error may be up to 20 ppm or more.
It is therefore a particularly attractive proposition to compensate for the effect of temperature on the time base frequency, in order to produce more accurate watches. To that end three main systems have been used hitherto, all of which employing two quartz resonators.
FIGS. 1a to 1c illustrate the basic principles of the three systems. In FIGS. 1a and 1b, the two oscillators each comprise an independently operating quartz resonator.
The difference between these two systems results from the thermal characteristics of the two resonators. The temperature T is shown along the abscissae and the relative variations in frequency are shown as the ordinates. In the process shown in FIG. 1a, the first resonator has a characteristic I such that its inversion point I.sub.1 coincides with a point on the characteristic II of the second quartz resonator. The second resonator has an inversion point I.sub.2 which does not play any particular part, in itself. The temperature coefficients of the second order (.beta.), that is to say, the coefficients of the terms in T.sup.2 in the formula which gives the variations in frequency in dependence on the variation in temperature, must be strictly equal. It is therefore necessary to use two resonators which are strictly paired, in order to provide an attractive compensation effect.
In the compensation system shown in FIG. 1b, the characteristics I and II of the two resonators must have coincident inversion points I.sub.1 and I.sub.2, that is to say, for the two resonators, the inversion temperature and the inversion frequency must be the same. In addition, the coefficient .beta.1 of the first resonator must be double the coefficient .beta.2 of the second resonator. It will be readily seen that in this case also it is necessary for the two resonators to be very carefully matched.
In the compensation mode illustrated in FIG. 1c, the two resonators are mounted in parallel in the same oscillator. In order to achieve good compensation in a given operating temperature range, the temperature and the frequency of the inversion points I.sub.1 and I.sub.2 of the characteristics I and II of the two resonators must be known perfectly. The two quartz resonators therefore also have to be matched, with a high degree of accuracy.
It will be seen from the foregoing consideration of the state of the art that, in all cases, it is only possible to produce an acceptable compensation effect, if two quartz resonators having thermal characteristics which actually comply with the conditions required are used in the same watch. It will be readily appreciated that selecting the two resonators results in a very substantial increase in the cost of the time base, because of the necessity for matching the two quartz resonators.